1. Field of the Invention
The present invention relates to a method of and a system for monitoring the operation of a beam steering unit for a phased array antenna, during a scanning operation of the beam steering unit. In particular, according to the invention, the pattern of wave energy which would be radiated from the antenna to an observation point in space during the scanning operation is simulated by processing phase angle data provided by the beam steering unit and combining it with observation angle data corresponding to the observation point.
2. Description of the Known Art
In order to verify proper operation of a beam steering unit associated with a scanning phased array antenna, it has ordinarily been required to monitor the wave energy actually radiated by the antenna to near and/or far observation point, and then compare the monitored energy levels with a reference standard. For example, in U.S. Pat. No. 4,520,361 issued May 28, 1985, to R. F. Frazita and assigned to the assignee of the present invention, phase angle data provided from a beam steering unit to each of a number of radiating elements of a phased array antenna, is verified separately for each of the elements by coupling some of the element radiation to a manifold at the antenna, mixing the manifold output with a sample of the RF power source to obtain a beat frequency signal, and measuring the phase shift between the beat frequency signal and a reference pattern signal.
U.S. Pat. No. 4,536,766 issued Aug. 20, 1985, to R. F. Frazita and assigned to the assignee of the present invention, discloses a beam pointing correction arrangement which also entails the use of a manifold proximate the radiating elements of a scanning phased array antenna, wherein the manifold output is detected and decoded to provide an indication of the actual beam pointing angle. The start and stop time of the beam steering unit scanning operation is then adjusted to eliminate or minimize any detected beam pointing error. A system is also known from U.S. Pat. No. 4,532,517 issued July 30, 1985, in which output data from a beam steering unit is subjected to a cyclic redundancy check employing algebraic methods commonly used to verify accuracy of information transmitted in digital form.
As far as is known, no method or system has been disclosed by which the pattern of wave energy radiated from a phased array antenna to an observation point during operation of an associated beam steering unit, can be simulated to allow for a comparison with a standard reference pattern. The desirability for such a method or system is especially great in microwave landing systems (MLS) in which precise timing of the beam steering operation must be maintained continuously to assure that an aircraft at a certain point in space relative to the system antennas will receive the antenna beams at the proper timings as the antenna beams are scanned "to and fro" and "up and down".
Basically, a MLS employs at least two phased array antennas each having a number of equally spaced radiating elements which are excited with microwave energy at a generally uniform amplitude but at a phase determined by the setting of individual phase shifters associated with the elements. The function of setting the phase shifts for the individual phase shifters is accomplished by the beam steering unit (BSU). As is well-understood by those skilled in the art, a main energy beam which is radiated from the excited antenna elements can be steered or scanned in a direction relative to the antenna, in accordance with predetermined incremental changes of the phase shifters by the BSU over successive time intervals.
In MLS applications, an azimuth (AZ) phased array antenna scans its radiated beam to and fro periodically in the horizontal direction, the beam-width being relatively broad in the vertical direction but narrow in the horizontal direction, so that an aircraft within the scanning Field of the AZ antenna will be able to detect a passage of the scanning beam from the AZ antenna from ground level to a relatively high altitude. An elevation (EL) phased array antenna scans its beam up and down periodically in the vertical direction, the beam width being relatively broad in the horizontal direction but narrow in the vertical direction, so that an aircraft within the scanning field of the EL antenna will be able to detect the passage of the scanning beam from the EL antenna from an approach which is head-on to the antenna to one which is about .+-.40.degree. relative to the antenna axis.
Prior to a scanning operation of the AZ antenna, a "preamble" signal is radiated broadly from a third antenna for reception by an aircraft within the operating range of the MLS. The preamble signifies, inter alia, that a horizontal scan of the beam from the AZ antenna is to begin at a certain time from one side (e.g., -40.degree.) of the AZ antenna, to the opposite side (+40.degree.), and back again to the starting side (-40.degree.). Equipment on board the aircraft detects and decodes the preamble, and counts the time period between reception of the beam from the AZ antenna on its "to" scan and reception of the beam on the "fro" scan. The counted time difference corresponds to a unique azimuth heading of the aircraft relative to the AZ antenna. The MLS then broadly radiates a preamble signifying that a scanning operation of the EL antenna is about to begin and, by a corresponding time difference counting operation, the equipment on board the aircraft determines a unique elevation angle for the craft relative to the EL antenna. Since both the AZ and EL antennas are located in the vicinity of a runway employing the MLS, the aircraft pilot thus receives information which is critical to assure a proper glide path for a safe landing on the runway.
From the foregoing, it will be appreciated that precise timing of the scanning operations of both the AZ and EL antennas is essential to ensure accurate glide path information will be provided to the aircraft pilot. Any malfunction which results in a deviation of the time difference between to and fro or up and down scanning beams at a given point in space, from a predetermined difference which defines the location of the point in space when the MLS is functioning properly, will cause the on-board equipment to produce erroneous heading information.
A major source of such potential system malfunction is the BSU which controls the direction and rate of scan of the beams from the AZ and EL antennas in the MLS. Thus, it is imperative that the BSU be monitored continuously with respect to the phase angle data which it provides to the phase shifters associated with the antenna elements, causing the beams to be swept at the desired predetermined rates.